It is known to concurrently provide feedback air/fuel control and monitor catalytic converter efficiency in response to exhaust gas oxygen sensors positioned upstream and downstream of the converter. In one known approach, converter degradation is indicated when a ratio of downstream to upstream sensor amplitude exceeds a reference value during a predetermined time period.
The inventors herein have recognized numerous problems with prior approaches. For example, variations in an operators driving habits over the predetermined time period may result in variation of test results and erroneous failure indications. An additional problem recognized by the inventors is that the sensor output amplitude may be clipped during transient air/fuel operation resulting in erroneous test indications. Further, the peak-to-peak sensor output may remain substantially unchanged with degradation in converter efficiency even though the amplitude in a rich or lean indicating direction may become degraded.